Duoprop sizes

[whisper]

Having looked at the Volvo Penta site it would appear that we could, with our KAD44, use either B3 or B4 sizes. The top speed range for each would be 28-31kts and 31-34kts respectively.
Any thoughts on what the difference would be on fuel consumption generally and boat speed at tick-over? We've currently got B4s, I think, and at tickover we run at a very nice (usually) controllable 3kts.
Over the year we average just over 3nm/gal.

I appreciate that these are probably simple questions with very complicated answers. It's just a general feel of the pros and cons of the different sizes that I'm after.

Thanks in anticipation,
John

 

[whisper]

Blimey, where have all the experts gone when you need them /forums/images/graemlins/confused.gif
I'd better post this on Scuttlebut /forums/images/graemlins/grin.gif

 

[bbriansim]

Hi it depends on what revs you are pulling if you are getting near max 3800/3900 you would over rev loose fuel consumption and drop a coulpe of knots going down in pitch, if you are down on revs 3600/3700 it would pull your revs back come out of the water better and poss a little more speed. I have AD41s in a Targa 31 and have propped down one size, comes out quicker cruises well and fine on fuel cruising at 24knts ,but down on top speed a couple of knots on top speed about 31/32. Hope this helps

 

[bbriansim]

The speed you have quoted is about right down a couple of knots with B3s, fuel very little diff rev slightly higher at say 22knots but less load, tickover speed I dont think noticable diff.

 

[Chris_d]

I think the difference to fuel consumption will be unmeasurable, all you will do is play with load to fuel curves and come out about even, maybe with reduced acceleration but slightly higher cruise speed under greater load so everything will balence out, depends what you want. At tickover you may as well be turning a couple of old spades for all the difference the pitch will make.

 

[gjgm]

if tic over gives 3 knots, I think you need to change props, John. You should be looking at double figures minimum /forums/images/graemlins/grin.gif

 

[whisper]

STUPID BOY!!!!!!

 

[whisper]

Thanks very much for the replies y'all (except Gavin /forums/images/graemlins/wink.gif) - changing over would not be as dramatic as I thought it might be.
Cheers
John